The present invention relates to an optical communication field. More particularly, the present invention relates to an optical module using a gradient index rod lens, which module is used, for example, in an optical multiplexer for combining signals, an optical demultiplexer for separating signals, a demultiplexer/multiplexer in a narrow sense for combining and separating signals, and a demultiplexer/multiplexer such as an optical branching and coupling device.
Such an optical module is disclosed in Japanese Laid-Open Patent Publication No. 2002-156547. As shown in FIG. 8, the optical module of the publication includes two optical fibers (single mode optical fibers) 11, 12, and a gradient index rod lens 13. A mirror 14 is provided at a position that is separated from the rod lens 13 by a predetermined distance L. A left end surface 13a of the rod lens 13 is ground to be inclined at a predetermined angle (for example, 8°) relative to a plane perpendicular to an optical axis C. A right end surface 13b of the rod lens 13 is ground to be flat and perpendicular to the optical axis C. The two optical fibers 11, 12 are supported by a capillary 15. The end surfaces of the optical fibers 11, 12 and the end surface of the capillary 15 are ground to be inclined like the left end surface 13a of the rod lens 13.
The optical fiber 11 is used to let light reach and enter the rod lens 13, and is referred to as an incoming optical fiber. The optical fiber 12 is used to transmit light from the rod lens 13, and is referred to as an outgoing optical fiber. Outgoing light from the incoming optical fiber 11 is focused by the rod lens 13 and the light reaches the mirror 14. Light reflected by the mirror 14 is focused by the rod lens 13 and the light is coupled to the outgoing optical fiber 12.
When such an optical module is used in a demultiplexer/multiplexer, a gradient index rod lens 13 is used having a lens length Z of 0.25 pitch or 0.23 pitch. In this case, the distance L between the right end surface 13b of the rod lens 13 and the mirror 14 is relatively short, 0 to 0.5 mm. “Pitch” refers to a period of meandering of a beam in a rod lens and is represented by P in the following description.
When the distance L needs to be extended, for example, to 1 mm in a conventional optical module, the rod lens 13 needs to be a lens that has a lens length Z of less than 0.23P. However, if such a rod lens, which has a short pitch and a ground inclined end surface, is used, an insertion loss is increased. “Insertion loss” is represented by the following equation.Insertion loss (dB)=−10×Log (Pout/Pin)
in which Pout represents the amount of outgoing light, and Pout represents the amount of reflected return.
The insertion loss is increased because, if the distance L is extended, the space WD between the rod lens 13 and the optical fibers 11, 12 is increased, and the influence of astigmatism and coma aberration due to the inclined end surface (the left end surface 13a) of the rod lens 13 is increased. The left end surface 13a of the rod lens 13 is machined to be an inclined surface so that light reflected by the left end surface 13a does not return to the incoming optical fiber 11. That is, the left end surface 13a is inclined to decrease reflected return light from the left end surface 13a. 